1. Field of the Invention
This invention relates generally to X-ray generators, and is concerned more particularly with an X-ray tube having a rotating anode provided with a peripheral groove focal track.
2. Discussion of the Prior Art
Generally, an X-ray tube of the rotating anode type comprises a tubular envelope having therein an anode target disc which is axially rotatable and provided with a radially sloped, annular focal track adjacent its periphery. A rectangular focal spot area disposed radially on the focal track usually is axially aligned with a linear filamentary cathode, and is radially aligned with an X-ray transparent window in the tube envelope. Due to the rotation of the anode target disc, the material of the focal track in the focal spot area is constantly changing in order to provide greater heat dissipation than X-ray tubes of the stationary anode type.
In operation, the cathode thermionically emits electrons which are electrostatically beamed onto the focal spot area with sufficient energy to generate X-rays. A useful portion of the X-rays radiating from the focal spot area pass in a divergent beam from the tube through the X-ray transparent window in the tube envelope. However, because the window is radially aligned with the focal spot area, the X-ray beam appears to be emanating from a radial projection of the focal spot area, which generally is referred to as the "effective" focal spot of the tube.
An edge portion of the beam emanating from the "effective" focal spot extends along the sloped surface of the focal spot area and, consequently, acquires a number of characteristics traceable to what may be termed as the "heel effect". For example, this edge portion of the X-ray beam, as compared to other portions thereof, appears to be emanating from a focal spot of radically different size and configuration, thereby degrading uniformity of resolution in a radiograph produced by the X-ray beam. Also, due to the filtering properties of the sloped target surface, which increase rapidly with aging, the adjacent edge portion of the beam has a lower X-ray intensity and a higher percentage of "hard" X-rays than other portions of the X-ray beam. As a result, the aligned portion of the radiograph exhibits a different quality of definition and contrast as compared with other portions of the radiograph.
Therefore, it is advantageous and desirable to provide an X-ray tube of the rotating anode type with an anode target having focal track means for producing an X-ray beam which does not have the undesirable characteristics traceable to the roughened surface of conventional anode target focal tracks.